Area Forecast Discussion
Issued by NWS Topeka, KS

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FXUS63 KTOP 240446
AFDTOP

AREA FORECAST DISCUSSION
National Weather Service Topeka KS
1146 PM CDT SAT APR 23 2016

...Update to aviation forecast discussion...

.SHORT TERM...(This Evening through Sunday)
Issued at 341 PM CDT SAT APR 23 2016

Latest water vapor satellite loop shows an upper level shortwave
trough over the Great Basin moving into the Rockies. This evening
the low level jet will increase from the south with moisture axis by
late evening west of the the CWA. Moisture and instability will
increase after 05Z as the low level jet veers to the southwest
through the night. Elevated thunderstorms will be possible across
north central Kansas late tonight and have made some adjustments on
the eastward extent of any precipitation chances. High cirrus will
spread from west to east through the night. Forecast soundings and
RH progs do no not show any low level stratus forming in the
moisture axis overnight. In addition the lower boundary layer will
be mixed and with increasing high clouds expect temperatures to only
fall to around 60 degrees for lows tonight.

Attention turns to Sunday where there may be ongoing convection from
the overnight hours moving mainly across north central and northeast
Kansas. There should be a lull from mid morning through early
afternoon. However the aforementioned upper trough will move out into
the plains with a shortwave rotating out of the base of the trough
from the Rockies northeast across Kansas taking on a slight negative
tilt. At the same time the dryline will mix east into central
Kansas and looks to set up west of U.S. 81 Highway by late
afternoon. Shear and instability will be favorable for discrete
supercell thunderstorms to develop along the dry line. Helicity in
the 0-1km and 0-3km layers are favorable for tornadoes during the
late afternoon hours. Large hail and damaging winds will be the main
hazards, with tornadoes possible with any discrete supercell. High
temperatures on Sunday will range from the upper 70s to lower 80s.
A tight pressure gradient will keep winds gusty across the area
again on Sunday.

.LONG TERM...(Sunday Night through Saturday)
Issued at 341 PM CDT SAT APR 23 2016

Sunday Night through Tuesday...

An upper level trough will lift east-northeast across eastern KS
Sunday night. Thunderstorms will continue along and east of the
dryline due to surface convergence ahead of the dryline and larger
scale ascent ahead of the H5 trough. MLCAPES will range between1500
and 2500 J/KG across the central counties of the of the CWA. The
0-6 KM effective shear will increase across the northern counties
to 40 to 55 KTS through the early evening hours. The combination
of CAPE and vertical windshear will create a favorable environment
for severe thunderstorms. Most numerical models show a line of
storms developing across the northeast and central counties of the
CWA. There may continue to be embedded supercell thunderstorms
within the line of storms but areas north of I-70 may see more of
a widespread damaging wind event during the evening hours. From
Abilene to east of MHK may see more discrete supercells extending
southwest to east of ICT. These storms will be capable of
producing large hail and localized damaging wind gusts. By 3Z MON
the 18Z NAM model backs the surface winds across the eastern half
of the CWA. This increased the 0-1 KM SRH to 200 to 300 J/KG and
would make the environment more favorable for any discrete
supercell to produce isolated tornadoes. However, as the boundary
layer cools during the evening hours, the tendency will be for
discrete supercells to be come more elevated. But there may be a
window from 7 PM to 9 PM where discrete supercells may have the
best environment to produce isolated tornadoes.

After 6Z MON the stronger ascent ahead of the H5 trough will shift
northeast of the CWA and the thunderstorms will diminish in
intensity across the CWA and shift northeast of the CWA. During the
early morning hours of Monday a weak cold front will over take the
dryline and shift southeast across the CWA. There could be enough
surface convergence for an isolated shower or thunderstorm ahead of
the cold front.

Monday through Monday night. Most models show the surface cold front
pushing southeast across the extreme southeast counties of the CWA.
Frontolysis will occur along the front and the front will become
very weak diffuse across southeast KS. A down stream upper ridge
will amplify across the plains as an intense upper low digs
southeast into the four corners area. I don`t expect much in the way
of any isolated showers or thunderstorms Monday and through most of
Monday night. I kept a slight chance for showers and thunderstorms
in late Monday night as deeper moisture will begin to return
northward across central KS. So, there could be some weak isentropic
lift for an isolated shower or elevated thunderstorm. Highs Monday
will be slightly cooler with mid to upper 70s.

Tuesday, this could be an interesting day across the plains by
afternoon. The ECMWF looks to be too robust with its QPF forecast
for the morning hours as subsidence will continue under the
downstream H5 ridge across the plains during the morning hours.

The 18Z NAM models is the slowest lifting the upper low across the
four corners region into the central high plains. The NAM models
shows an EML over spreading the CWA with a strong CAP across much of
the CWA through 00Z WED. The NAM does punch the dryline east into
south central and central KS by 00Z WED, but has no surface based
convection during the day. The surface warm front will lift
northward to the NE border by the late afternoon hours. The main
upper trough will be lifting northeast into CO and norther NM which
will keep the stronger ascent across western KS. The H5 jet max of
50-55 KTS will lift northeast across west central KS and much of the
CWA will be on the subsidence east side of the H5 jet. However, if
temperatures get close to 80 degrees along the dryline there may be
enough convergence for an isolated supercell thunderstorm to develop
and move northeast off the dryline towards Ottawa and Cloud Counties
during the late afternoon hours. The NAM had over 4,000 J/KG of
MLCAPE and stronger vertical windshear forecasted across the western
counties of the CWA. Thus if a storm were to develop it would quickly
develop into a supercell that would produce large hail, local
damaging wind gusts and tornadoes.

 The GFS model does not show as stout of an EML over spreading the
area, which would cause a weaker CAP to develop during the afternoon
hours. The GFS moves the dryline east of I-135/US 81 by late
afternoon with the warm front along I-70. The GFS develops QPF along
and south of the warm front across much of east central KS during
the afternoon hours. If the GFS were to verify there would be a much
better chance for surface based storms to develop during the mid and
late afternoon hours. The MLCAPE is progged to be in the 2500-4000
J/KG range across the CWA. Given the high CAPE and stronger
vertical windshear, initial supercell thunderstorms will produce
large hail, damaging winds and tornadoes. The storms may become more
widespread towards 00Z WED, which may cause line segments of storms
to develop. The main threat from any line segment of severe
thunderstorms would be widespread wind damage and the potential
for meso vortices tornadoes within the line of storms.

At this time based on the slightly slower progression of the upper
level trough I`m leaning more towards the 18Z NAM solution. But
severe storms will develop during the evening hours as the stronger
ascent ahead of the H5 trough spreads east across the CWA. Everyone
should pay attention to the latest forecast on Tuesday afternoon and
evening since the combination of high CAPE and stronger vertical
windshear may cause some of the supercells late Tuesday afternoon
and evening to produce stronger tornadoes in or near the CWA.

Tuesday Evening through Saturday...

Complicated severe weather setup for Tuesday evening into early
Wednesday morning. 12Z models continue to slow the progression of
the upper level trough across the central plains. At 00Z Wednesday,
good model agreement amongst ECMWF, GFS and NAM with the placement of
the 500mb wave across western Kansas. It appears models generally
agree upon the dryline and warm-front placement, with the dryline
along the Interstate-135/Highway-81 corridor and the warm-front near
the KS/NE border. Areas within the warm sector shall see dewpoints
in the low to mid 60s by Tuesday afternoon. The biggest unknown at
this point is storm initiation ahead of the main wave. Current
forecast soundings continue to show a strong enough EML to keep
surface based convection minimized prior to the upper-level support.
With that being said, ML CIN along the dryline is currently
forecasted to be near 50 J/KG. This minimal CIN may be overcome by
surface convergence along the dryline, allowing isolated supercells
to develop. "If" this scenario were to come to fruition, all modes of
severe weather would become likely as moderate/strong instability and
impressive shear profiles are expected to be in place. Will have to
continue to monitor the possibility for development along the warm
front, although an even stronger capping inversion is expected.

Higher confidence in thunderstorm activity late Tuesday night into
Wednesday morning as the main upper level trough ejects across the
central plains. Elevated supercells will be possible as a 40-50 knot
LLJ and 60-70 knot 500mb jet overspread the area. The main threat
with these storms will be very large hail and gusty winds.

Another trough is expected to eject into the central plains for
Friday and Saturday.

&&

.AVIATION...(For the 06Z TAFS through 06Z Sunday Night)
Issued at 1146 PM CDT SAT APR 23 2016

VFR conditions should be the rule for this forecast. Convective
trends to the west suggest a bit earlier potential for high-based
precip at MHK. Persistence and timing of convection here is
challenging both early and late in the period and no doubt
alterations will be needed in subsequent forecasts.

&&

.TOP Watches/Warnings/Advisories...
NONE.
&&

$$

SHORT TERM...53
LONG TERM...Baerg/Gargan
AVIATION...65



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